9: INLETS AND DIFFUSERS

In the following sections, we will consider the design considerations of each of the components of engines. This section will draw upon the fluid dynamics and combustion sections of the course, and consists of discussion of several engine types and missions.

The function of an inlet and diffuser is to decelerate the flow coming into the engine with minimum loss in stagnation pressure. Associated with this are other design criteria such as the need to accomodate a large range of variation in mass flow, as engine demands change, and the need to operate without flow distortion or separation at large angles of attack. We will consider the design issues of subsonic and supersonic diffusers separately, recognizing that all supersonic diffusers must be followed by a subsonic diffuser.

Subsonic Diffuser

Here the design problem is to decelerate flow in the minimum possible length without causing boundary layer separation and with minimum external drag.

Note:

1. In the diffuser, there is an adverse pressure gradient, so that the boundary layer thickens rapidly and becomes unstable.

2. The mass flow rate and the downstream pressure are determined by engine operating conditions: usually by the conditions at the choked turbine exit, and by the operating speed of the compressor and turbine.

Because of this variation in mass flow requirements, the inlet may be operating under suction (external acceleration) or spillage (external deceleration).

External Deceleration (Spillage)

Here the engine cannot ingest all of the mass flow in the area Aa upstream:

some air is pushed aside.

This occurs usually at high flight speeds (cruise). It is a favorable condition for the diffuser because the pressure rise in the diffuser is smaller. Thus losses are smaller. But external drag is increased.

b) External Acceleration

Here the engine demands mass flow: the pressure at the inlet face is lower than the ambient pressure, so that air is sucked into the inlet. This usually occurs at low velocity, with the engine at high thrust, for example at takeoff.

The disadvantage is that the diffuser now has to work harder, and the adverse pressure gradient is worse: boundary layer separation and compressor stall become more likely. Many aircraft use "bypass doors" which open at takeoff to pull in more air. In high-speed flight, these doors may slide open to let out some of the excess air and thus reduce spillage around the lip of the inlet.

Note: Usually there is a stagnation pressure loss of 5 to 10% in the subsonic diffuser, so diffuser design is crucial to the efficiency of the engine.

Supersonic Inlets

In addition to friction (boundary layer) losses, the supersonic diffuser must be designed to minimize losses in stagnation pressure across shocks. The ideal supersonic diffuser is one which decelerates the flow until it reaches Mach 1.0 at the throat, with no shocks, in an isentropic process. This requires a continuously-varying shape, which is complex to manufacture. Even if this were possible, such a design would be isentropic for only one upstream Mach number and angle of attack: at other conditions, strong shocks may form.

Since shocks will occur in any practical supersonic diffuser, we recall that the stagnation pressure loss across a shock increases as the Mach number of the velocity component normal to the shock increases. Thus, the loss can be reduced by making the first shock oblique. Also, the flow must go through a normal shock of some finite strength before it becomes subsonic. Thus, the design problem becomes that of finding the optimum combination of oblique shocks and normal shock which will provide the lowest loss in stagnation pressure.

Note:

Given N-1 variable ramps, to produce N-1 oblique shocks of controllable strength, and one normal shock, it can be shown that the minimum loss in stagnation pressure occurs at approximately the condition where the normal Mach number in front of every shock is the same.

For aircraft which fly at low supersonic Mach numbers, only a normal shock inlet is used. For higher Mach numbers, as many as three variable ramps have been used.